Versatile channel selection procedure for wireless network

ABSTRACT

A method of selecting a frequency block includes determining a first value representing a lowest noise level of a radio channel at a first or second node, calculating a first metric representing a ratio between a first desired signal strength and the first value, determining a second value representing a lowest noise level of a radio channel at the first or second nodes, calculating a second metric representing a ratio between a second desired signal strength and the second value, selecting, on the basis of at least the first and second metrics and amongst a set of frequency blocks including at least the first and second frequency blocks, a frequency block for transmission of a message between the first and second nodes, indicating a sub-band of the selected frequency block, and causing transmission of the message on the indicated sub-band.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/834,771, filed Dec. 7, 2017, which claims benefit to EuropeanApplication No. 16203397.1, filed Dec. 12, 2016, which are incorporatedby reference herein in their entireties.

BACKGROUND Field

The invention relates to the field of long range radio communicationsand, particularly, to carrying out a frequency channel selectionprocedure in a radio communication apparatus.

Description of the Related Art

Modern radio communication systems support operation on a frequencychannel selected from a plurality of frequency channels according to adetermined criterion. Some systems rely on frequency planning where agiven frequency band is assigned to the system, and the system isconfigured to operate exclusively on that frequency band. Such systemsare typically based on using licensed frequency bands. Other systems areconfigured to choose a frequency to be used more adaptively, e.g. on thebasis of scanning for the available (non-occupied) frequencies and,then, transferring control messages related to negotiation of thefrequency band to be used. Such methods increase signalling overhead,particularly in networks comprising numerous network nodes.

In some systems, a channel is selected for communication afterperforming channel measurements. The measurements are typically based ona first node transmitting a pilot signal to a second node on a channeland the second node measuring the pilot signal. Such measurements may becarried out for multiple frequency channels and, after the measurements,the nodes may select a channel for bidirectional communication. Such aprocedure is applicable to systems where the nodes experience asubstantially similar radio environment. In long range communicationsystems, two geographically distant nodes may experience completelydifferent radio environments, and many conventional channel selectionprinciples are not applicable. Similar phenomena may be experienced evenin short and medium range radio communications.

SUMMARY

The invention is defined by the subject-matter of the independentclaims.

Embodiments of the invention are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described below, by way ofexample only, with reference to the accompanying drawings, in which

FIG. 1 illustrates a wireless network to which embodiments of inventionmay be applied;

FIG. 2 illustrates a messaging scheme to which some embodiments of theinvention may be applied;

FIG. 3 illustrates a frequency channel structure to which someembodiments of the invention may be applied;

FIG. 4 illustrates a flow diagram of a process for selecting a frequencyblock and a sub-band for transmission of a message according to anembodiment of the invention;

FIG. 5 illustrates an example of noise levels in a frequency block;

FIG. 6 illustrates an embodiment for computing, by an apparatus,potential signal-to-noise ratios for frequency blocks and selecting afrequency block for a message to be received at the apparatus accordingto an embodiment of the invention;

FIG. 7 illustrates an embodiment for computing, by an apparatus,potential signal-to-noise ratios for frequency blocks and selecting afrequency block for a message to be transmitted from the apparatusaccording to an embodiment of the invention;

FIG. 8 illustrates an embodiment combining the embodiments of FIGS. 6and 7;

FIGS. 9A to 9C illustrate embodiments of messages exchanged in theembodiments of FIGS. 4 and 6 to 8;

FIG. 10 illustrates an embodiment for versatile computation of thepotential signal-to-noise ratios for multiple frequency blocks; and

FIG. 11 illustrates a block diagram of a structure of an apparatusaccording to an embodiment of the invention.

DETAILED DESCRIPTION

The following embodiments are exemplary. Although the specification mayrefer to “an”, “one”, or “some” embodiment(s) in several locations, thisdoes not necessarily mean that each such reference is to the sameembodiment(s), or that the feature only applies to a single embodiment.Single features of different embodiments may also be combined to provideother embodiments.

Furthermore, words “comprising” and “including” should be understood asnot limiting the described embodiments to consist of only those featuresthat have been mentioned and such embodiments may contain alsofeatures/structures that have not been specifically mentioned.

FIG. 1 illustrates an exemplary wireless telecommunication system towhich embodiments of the invention may be applied. Embodiments of theinvention may be realized in an ad hoc network comprising a plurality ofnetwork nodes 10, 11, 12 that may be realized by radio communicationapparatuses. The ad hoc network may refer to a network that isestablished between the network nodes 10 to 12 without any networkplanning with respect to the infrastructure and/or frequencyutilization. The network nodes may be operationally equivalent to eachother although in some embodiments some of the network nodes of thesystem may operate as access nodes to other network nodes. The termaccess node may refer to a network access service provided by an accessnode, wherein the access node provides another network node with radioaccess to at least one network, e.g. a local area network, a wide areanetwork, and/or the Internet. At least some of the network nodes 10 to12 are free to move, and they may also be configured to route datapackets that are unrelated to their own use, e.g. data packets of othernetwork nodes. However, it should be understood that principles of theinvention may be applied to other types of communication systems, e.g.wireless mesh networks, communication systems having a fixedinfrastructure such as cellular communication systems, and other typesof systems. The principles of the invention may also be applied topoint-to-point connections, wherein two network nodes communicationdirectly with each other without using any other network node to routethe data packets.

In the embodiment of FIG. 1, the network nodes 10 to 12 have a very longcommunication range (even thousands of kilometres), and they maycommunicate directly with network nodes on the other side of the Earth.Their transmit powers may vary from a few Watts (e.g. 20 to 50 W) toeven kilo Watts, depending on whether the network node is mobile orfixed and the type of power supply. For example, a network nodeinstalled to a building, a truck, or a ship may utilize high transmitpowers, while a hand-held device may be limited to a few Watts. Thefrequency band utilized by the network nodes 10 to 12 may comprise ahigh frequency (HF) band (3 to 30 MHz or 1.5 to 30 MHz), but it shouldbe understood that other embodiments utilize other frequency bandssupporting such long-range radio links, e.g. very high frequencies (VHF)or ultra-high frequencies (UHF). An advantage of HF frequencies is theirlong propagation range, and the fact that they may propagate via severaltypes of communication paths. FIG. 1 illustrates a scenario where afirst network node 10 communicates with a second network node 11 oversurface radio waves that propagate close to the ground surface. However,a third network node 12 on the other side of the Earth may be reached bythe first network node 10 via radio waves that propagate by utilizingionospheric reflections. Some network nodes may be reached by using bothsurface waves and ionospheric reflections. The network nodes 10 to 12are configured to support communication on a high frequency band fromwhich actual transmission frequencies may be selected according toembodiments described herein. The supported frequency band may becontinuous or divided into a plurality of frequency bands separated fromeach other. The division may be based on the fact that there are othersystems occupying some frequencies that may have a priority to occupythe frequencies, while the present system has to adapt to the frequencyoccupation of such a primary system. In some embodiments, the systemsoccupying the same frequency band have equal priority to the frequencyoccupation, and at least the present system may utilize cognitivechannel selection procedures described herein to avoid collisionsbetween the systems.

FIG. 2 illustrates a flow diagram of a process for a procedure to whichsome embodiments of the invention may be applied. FIG. 2 illustrates ahandshake procedure in which two network nodes 10 and 12 initiate datatransmission. The handshake procedure in this embodiment involvesexchange of request-to-send (RTS) and clear-to-send (CTS) messages.Referring to FIG. 2, the network nodes may be configured to exchangenarrowband control messages with one another in S10. Each controlmessage comprises a pilot sequence and an identifier identifying atransmitter of the control message. In some embodiments, the controlmessage consists of the pilot sequence and the identifier. The exchangeof the control messages may be carried out repeatedly according topreset rules that may be time-based and/or need-based. The controlmessages may be considered as advertisement or discovery messages withwhich the network nodes indicate their presence in the network and,additionally, indicate preferred frequencies for communication, asdescribed below.

With respect to reception of a single narrowband control message in anetwork node 10 to 12, the network node may receive a broadband signalthrough a broadband radio receiver. The broadband radio receiver may beconfigured to carry out the reception on a frequency band that issignificantly broader than a bandwidth of the narrowband controlmessage. In some embodiments, the bandwidth of the receiver may be morethan ten times the bandwidth of the narrowband control message, and inother embodiments even hundreds or thousands time the bandwidth of thenarrowband control message. The network node may then carry out a signaldetection procedure on the received broadband signal so as to detect anarrowband control message within the received broadband signal. Thesignal detection may be carried out for a plurality of sub-bands of thereceived broadband signal. For example, the received broadband signalmay be divided into a plurality of sub-bands having the bandwidthcorresponding to the known bandwidth of the narrowband control message,and the signal detection process may be carried out for each sub-bandseparately. In practice, the received signal of a given sub-band may becorrelated with a pilot sequence stored in a memory of the apparatus.The pilot sequence may be the same as the pilot sequence added to thenarrowband control message in its transmitter.

Upon detection of the narrowband control message on a sub-band of thereceived broadband signal, a transmitter of the narrowband controlmessage may be determined from the identifier comprised in the detectednarrowband control message. The network nodes 10 to 12 may be configuredto use the narrowband control messages to indicate preferred sub-bands,e.g. the sub-bands providing a communication quality high enough fromthe perspective of the transmitter of the narrowband control message. Inan embodiment, the transmitter may indicate the preferred sub-band bytransmitting the narrowband control message on the preferred sub-band.In another embodiment, the narrowband control message comprises aninformation element having a bit value that indicates the preferredsub-band(s), e.g. a bitmap described below. Therefore, a receiver of thecontrol message may determine from the received control message(s) thesub-band(s) preferred by the transmitter of the narrowband controlmessage.

The term “narrowband” may be defined with respect to the “broadband”such that the bandwidth of the narrowband control message is lower thanthe bandwidth of a broadband radio receiver. According to another pointof view, the narrowband may be defined with respect to its transmissionfrequency, e.g. the bandwidth of the narrowband control message is 10%or less than the centre frequency carrying the control message. On theother hand, the bandwidth of the broadband radio receiver is higher than10% of the centre frequency of the control message.

Channel selection is carried out in S11 by the network node 10 havingdata to transmit to the network node 12. The network node 10 may theninitiate a data transmission. Parameters of the data transmission may benegotiated in a negotiation phase in which the network node 10 transmitsa RTS message to the network node 12, and the receiver responds with aCTS message. In S12, the transmitter transmits the RTS message to thereceiver. The RTS message may specify an amount of resources needed fortransmission of the data, a quality-of-service classification for thedata, and/or other information that enables the network node 12 toallocate sufficient resources for data transmission. The RTS message mayfurther contain information that enables the network node 12 to select afrequency block for the CTS message. Embodiments of this feature aredescribed below. Upon reception of the RTS message in S12, the receiverdetects the RTS message in S13, determines a bandwidth needed for thedata transmission, and performs selection of one or more frequencyblocks for the data transmission.

In S14, the receiver prepares the CTS message for transmission to thetransmitter. The network node 12 may transmit the CTS message on asub-band of a frequency block indicated in the RTS message in S12, andthe CTS message may comprise a frequency block allocation for thesubsequent data transmission. In an embodiment, the CTS message may alsocomprise a sub-band allocation for frequency block(s) indicated in thefrequency block allocation.

In S15, the network node 10 carries out the data transmission in thefrequency resources allocated in the CTS message. The network node 12 isconfigured to monitor for those frequency resources for the datatransmission. Upon reception of the data transfer on those channels, thenetwork node 12 processes the received data by carrying out datadetection and decoding algorithms. Upon successful reception of thedata, the network node 12 is configured to transmit an ACK message in afrequency block specified in the RTS message in S12. However, uponerroneous reception of the data, the network node 12 is configured totransmit a NAK message in the frequency block specified in the RTSmessage in S12. In some embodiments, the network node 12 responds onlyto the correct reception (ACK) or to the erroneous reception (NAK) ofthe data. For example, when the network node 12 acknowledges only thecorrect receptions by transmitting ACK, the network node 10 detectserroneous reception upon detection of no ACK message for a given datapacket. Any hybrid automatic repeat request (HARQ) procedures are alsopossible, wherein upon detecting erroneous reception of a data packet, aretransmission comprises either the same data packet (chase combining)or additional information (e.g. parity bits) that help the decoding inthe receiver. The latter embodiment is known as incremental redundancyHARQ. In this manner, the data transfer may continue between the networknodes 10 to 12.

As a result of the above-mentioned channel selection procedure, nomanual frequency planning or excessive control signalling related to thenegotiation of the common frequency band(s) to be utilized in thecommunication is necessary. Repeated transmission of the controlmessages also enables fast adaptation to changing radio environment.Typically, one sub-band may have high quality for a given time periodafter which other systems occupy the sub-band, and the quality of thesub-band deteriorates. For example, HF frequencies are susceptible tovarious natural phenomena, e.g. solar activity and other radiationsoriginating from the space, and the other radio systems also contributeto the changing radio environment. Systems with static frequencyplanning cannot adapt to such changes and, therefore, their performancedegrades. Furthermore, the radio environment may be completely differentfor two network nodes far away from each other. This raises therequirements for the fast adaptation, as the probabilities that at leastone of two network nodes experiences degradation of current sub-bands isincreased. A network node may, upon detection of poor performance in thecurrently preferred sub-band(s) and/or frequency block(s), scan forbetter sub-bands and/or frequency blocks and transmit one or more newcontrol messages indicating an updated list of preferred frequencies.Upon reception of the new control message(s), the other network nodesmay update the preferred channel list accordingly. The channel selectionprocess comprising the exchange of the control message(s) and theprocessing of the received control message(s) may take even less than200 ms which enables fast adaptation to the changing radio environmentand may be carried out even without any negotiation other than theunidirectional transmission of the control message.

Let us consider a channel structure with reference to FIG. 3. FIG. 3illustrates that the operational band of the whole system is dividedinto a plurality of frequency blocks 300, 302, each frequency block 300,302 having an exemplary 192 kHz bandwidth. Each network node 10 to 12may be tuned to receive 192 kHz signals of each frequency block. Thenetwork node 10 to 12 may comprise a plurality of radio receivers,wherein each radio receiver is tuned to receive radio signals on atleast one frequency block. In some embodiments where the number offrequency blocks supported by the system is higher than the number ofradio receivers in the network node, at least some of the radioreceivers are tuned to receive a plurality of frequency blocks. Theradio receivers may then carry out frequency-hopping between saidfrequency blocks. The bandwidth of the actual transmissions is 3 kHz.Each 192 kHz frequency block is divided to 3 kHz (1 kHz or anotherbandwidth in other embodiments) sub-bands. In some embodiments, thenumber of sub-bands in the frequency blocks is the bandwidth of thefrequency block divided by the bandwidth of the sub-band, e.g. 192 kHz/3kHz=64. In such embodiments, the separation between centre frequenciesof adjacent sub-bands is equal to the bandwidth of the sub-bands, e.g. 3kHz. However, in more efficient embodiments, the separation betweencentre frequencies of adjacent sub-bands is lower than the bandwidth ofthe sub-bands. This effectively means that the sub-bands overlap in thefrequency domain, as illustrated in FIG. 3, but sufficient frequencyseparation may still be achieved so that adjacent channel interferencemay be mitigated in the receiver. For example, the centre frequencyseparation may be 1 kHz or even 500 Hz, while the bandwidth of thesub-band is several kHz. A network node 10 to 12 may be configured toselect one or more sub-bands per frequency block to carry out atransmission.

As the transmitter may select the sub-bands on which to transmit thecontrol messages, each broadband receiver do not necessarily know onwhich one of the sub-bands of the frequency block the transmission islocated. As a consequence, each receiver branch may comprise a matchedfilter matched to a waveform of a known pilot sequence contained in thetransmission and, further, be configured to scan for the (3 kHz)sub-bands of the received broadband (192 kHz) signal and to detect thepilot sequence. As known in the art, the matched filters may be replacedby a correlator structure.

Upon detection of the pilot sequence in one of the sub-bands of thereceived signal, a signal on the sub-band is applied to a controlmessage processor that may be configured to process the signal. Theprocessing may comprise applying receiver signal processing algorithms,e.g. equalization, to the sub-band signal. The pilot sequence containedin the received sub-band signal may be used as a training sequence forthe equalization (a channel response may be estimated from the pilotsequence) and for other signal processing algorithms. Then, the controlmessage processor may extract a payload portion of the control messagecontained in the sub-band signal and recover any payload contained inthe payload portion.

Some embodiments of the invention described below are applicable to thechannel structure of FIG. 3 and the messaging scheme of FIG. 2. FIG. 4illustrates a process for selecting a frequency block 300, 302 for radiocommunications, the method comprising in a first node 10 of a wirelessnetwork: determining (block 400) a first value representing a lowestnoise level of a radio channel at the first node or at a second node 11,12 in a first frequency block; calculating (block 400) a first metricrepresenting a ratio between a first desired signal strength and thefirst value, wherein the first metric represents the highest potentialsignal-to-noise ratio in the first frequency block; determining (block402) a second value representing a lowest noise level of a radio channelat the first node or at the second node in a second frequency block;calculating (block 402) a second metric representing a ratio between asecond desired signal strength and the second value, wherein the secondmetric represents the highest potential signal-to-noise ratio in thesecond frequency block; selecting (block 404), on the basis of at leastthe first and second metric and amongst a set of frequency blockscomprising at least the first and second frequency block, a frequencyblock for transmission of a message between the first node and thesecond node; indicating (block 406) a sub-band of the selected frequencyblock for the message; and causing transmission of the message on theindicated sub-band of the selected frequency block (block 408).

In an embodiment, the first desired signal strength is measured from afirst desired signal received in the first node 10, and the seconddesired signal strength is measured from a second desired signalreceived in the first node 10, wherein the first desired signal isreceived in the first frequency block and the second desired signal isreceived in the second frequency block different from the firstfrequency block.

In an embodiment, the process of FIG. 4 results in only one metriccalculated per frequency block, wherein the metric represents the ratiobetween the desired signal strength and the lowest noise level in thefrequency block. This metric thus represents the best potentialsignal-to-noise ratio (SNR) that can be achieved for a communicationsignal transmitted or received in the frequency block. The frequencyblock selection may thus be carried out by using these estimates of thebest potential SNR per frequency block, and the selection in block 404may include selecting the frequency block that provides the best SNRamongst the frequency blocks, i.e. selecting a frequency blockassociated with a metric indicating the best potential SNR. Theselection of a sub-band within the selected frequency block may becarried out by using another criterion, as described below.

FIG. 5 illustrates an example of channel conditions in a frequency block300 that may be measured in network nodes 10 to 12. The curve drawn witha solid line represents a frequency-dependent noise level 500. The noiselevel 500 may be affected by background noise (e.g. thermal noise) andother radio transmissions. The background noise may be spectrallysubstantially white and the noise level may be coloured by the otherradio transmissions in the system of the network nodes 10 to 12 and/orby other wireless systems. If there exists at least one radiotransmission in the frequency block 300 at the time of measuring thenoise level 500, the measured noise level 500 spectrum may be coloured,i.e. be spectrally non-flat. The number of radio transmissions in thefrequency block 300 is proportional to the volatility of the measurednoise level 500. The embodiment of FIG. 4 is particularly suitable forscenarios where the noise level 500 has a non-flat spectrum in thefrequency block 500 because it then enables detection of the bestpotential SNR achievable in the frequency block instead of justdetermining whether or not there is interference in the frequency block.Additionally, the embodiment of FIG. 4 enables estimation of the bestpossible SNR for the frequency block even when the desired signal is notreceived on a sub-band associated with the lowest noise level. In fact,the desired signal may be received on any one of the sub-bands of thefrequency block.

The lowest noise level 502 may be calculated, for example as follows. Inan embodiment, the lowest noise level is calculated for a frequencyblock 300 by first measuring the radio channel on a frequency band ofthe frequency block 300, thus acquiring a measured radio signal. Then,the measured radio signal may be transformed into a frequency domain bya (fast) Fourier transform, thus resulting in a set of frequencysamples. The Fourier-transformed signal may be processed to represent,and the set of frequency samples may represent a frequency response ofthe radio channel in the frequency block 300, an amplitude spectrum ofthe radio channel in the frequency block 300, or a power spectrum of theradio channel in the frequency block 300, or another representation ofmeasured frequency characteristics of the radio channel in the frequencyblock. The lowest noise level may be achieved by selecting the smallestvalue amongst the frequency samples or another, more sophisticated,algorithm for selecting the lowest noise level may be employed.

FIG. 5 also illustrates the desired signal level 504, i.e. the strengthof the desired signal. In an embodiment, the desired signal is measuredby the first node from a radio signal received from the second node. Inother words, the desired signal strength 504 represents a signal levelof a radio signal received at the first node from the second node. Thedesired signal strength may be measured by using state-of-the-artmeasurement solutions, e.g. measuring a received signal level from thecontrol messages exchanged in S10 in FIG. 2. The strength of the desiredsignal may represent only the strength of the desired signal, i.e.without the noise or interfering other radio transmissions affecting thestrength. The contribution of only the desired signal to the signalstrength (e.g. power) measured from a channel (e.g. a sub-band) may betaken into account by using conventional signal processing tools. Forexample, a signal strength of a signal received from the channel may bemeasured at first in a state where the desired signal is detected in thechannel. This measured signal strength includes the desired signalcomponent, a noise component, and an interference component. Second, thenoise and interference power are estimated in a state where the desiredsignal is not detected in the channel. The detection of thepresence/absence of the desired signal may be carried out by usingconventional channel sensing mechanisms. In the third step, these twomeasurement values are subtracted such that the result is the desiredsignal strength. The order of the first and second step is naturallyinterchangeable. The first and second step may be carried out within adetermined time window during which the noise and interferencecomponents may be assumed as substantially constant or sufficientlystatic.

In an embodiment, the strength of the desired signal is measured on afrequency different from the frequency of the lowest noise level 502,e.g. a frequency band of the lowest noise level 502 and a frequency bandwhere the desired signal strength 504 is measured may be exclusivelydifferent. This enables computation of the potential SNR for thefrequency block regardless of the frequency band used for measuring thedesired signal strength. This embodiment is described in greater detailbelow with reference to FIG. 10. Let us now consider some embodimentsfor carrying out the selection of the frequency block. FIGS. 6 and 7illustrate signalling diagrams of embodiments where the frequency blockis selected by using a recipient of the message of block 408 as afurther criterion. In particular, if the recipient is the first nodeexecuting the process of FIG. 4, the metrics may be computed by usingchannel measurements performed by the first node. This is described inconnection with FIG. 6 and, in this case, the first node may trigger thesecond node to transmit the message to the first node in block 408. Ifthe recipient is the second node, the metrics may be computed by usingthe lowest noise levels for the frequency blocks as received from thesecond node. This is described in connection with FIG. 7 and, in thiscase, the first node may transmit the message from the first node to thesecond node in block 408.

Referring to FIG. 6, the first node is the network node 10 and thesecond node is the network node 12. In block 600, the network node 10performs radio channel measurements and computes, on the basis of themeasurements, the lowest noise level values N_(k) ¹⁰ for each of aplurality of frequency blocks 1 to K, thus acquiring values N₁ ¹⁰ toN_(K) ¹⁰. Block 600 may be carried out regularly, e.g. periodically. Instep 602, the network node 12 transmits a radio signal in a frequencyblock k and the network node 10 receives the radio signal and measures areceived signal strength P_(k) ^(rx) of this radio signal upon receivingthe radio signal. The radio signal may carry a control message such asthat described above in connection with S10 of FIG. 2. The radio signalmay be transmitted on any sub-band of the frequency block k. The signalstrength may be measured from a pilot sequence part comprised in themessage. The received signal strength P_(k) ^(rx) may be a receivedsignal strength indicator (RSSI) that represents a power of the receivedradio signal in the frequency block k, although another metricrepresenting the signal strength may be used (e.g. amplitude or energy).Step 602 may precede block 600 in some cases. Block 602 may be carriedout for a plurality of frequency blocks in which the radio signal oranother radio signal from the network node 12 has been received. In acase where the network node 12 transmits the radio signal in everyfrequency block 1 to K, P_(k) ^(rx) may be computed by the network node10 for every frequency block 1 to K. In other embodiments, the networknode 10 may receive the radio signal in a subset of the frequency blocks1 to K and, then, the network node 10 may compute the P_(k) ^(rx) onlyfor the subset of frequency blocks.

In block 604, the network node 10 computes the potential SNR for eachfrequency block for which P_(k) ^(rx) has been measured and fortransmissions from the network node 12 to the network node 10 as:

${{SNR}_{1}^{{12}\rightarrow{10}} = \frac{P_{1}^{rx}}{N_{1}^{10}}},{{SNR}_{2}^{{12}\rightarrow{10}} = \frac{P_{2}^{rx}}{N_{2}^{10}}},\ldots\mspace{14mu},{{SNR}_{K}^{{12}\rightarrow{10}} = \frac{P_{K}^{rx}}{N_{K}^{10}}}$Then, the frequency block providing the highest SNR_(k) ^(12→10) may beselected in block 604. Upon selecting the frequency block, the networknode may generate a message for transmission to the network node 12 andinsert into the message an index of the selected frequency block toindicate to the network node the frequency block in which the networknode 12 shall transmit a subsequent message to the network node 10. Inan embodiment, the message generated and transmitted by the network node10 in step 606 is the RTS message (see S12 in FIG. 2). In anotherembodiment, the message generated and transmitted by the network node 10in step 606 is another request message or another message requiring aresponse from the network node 12. The message may be a data packetrequiring an acknowledgment (ACK/NAK), for example.

The network node 10 may further indicate with the message generated andtransmitted in step 606 a preferred or even a selected sub-band withinthe selected frequency block. In an embodiment, described in greaterdetail below, the network node 10 may insert into the message aninformation element indicating sub-band(s) within the selected frequencyblock. The network node may thus specify a sub-band and a frequencyblock in which it shall receive the response to the RTS message.Accordingly, the network node may include in the message separateinformation elements for the selected frequency block and for preferredsub-band(s) within the frequency block.

The RTS message may further include a bitmap that indicates preferredsub-bands for the frequency block in which the RTS message istransmitted. The bitmap may serve the same purpose as in the controlmessage of S10, e.g. as an indicator of one or more sub-bands in whichanother node can contact the network node 10.

Upon receiving the message in step 606, the network node 12 may generatea response to the message. The network node may select the frequencyblock indicated in the message received in step 606. The network node 12may further select a sub-band indicated as the preferred or selectedsub-band in the message received in step 606. If multiple sub-bands areindicated as preferred sub-bands, the network node 12 may select asub-band arbitrarily, e.g. randomly. If a single sub-band is explicitlyspecified in the message, the network node 12 is obliged to select thesub-band. The network node may transmit the response message in step 608on the selected sub-band of the selected frequency block. The responsemessage may be the CTS message described above in S14 or anacknowledgment message, for example.

Referring to the embodiment of FIG. 4, the indication of the sub-band inblock 406 is the indication of the preferred sub-band(s) in step 606,and block 408 may comprise the transmission of the message in step 606that triggers (causes) the transmission of the message referred to inblock 408. The message of block 408 may be the response to the messagetransmitted in step 606.

In the embodiment of FIG. 6, the network node 10 selects the frequencyblock for a message it is bound to receive from the network node 12.FIG. 7 illustrates an embodiment where the network node 10 selects afrequency block for a message it is about to transmit to the networknode 12. In this embodiment, the network node 10 uses information onchannel conditions (a noise level) in the other network node 12. Let usremind that the system is applicable to long-range communications and,accordingly, two distant network nodes 10 12 may observe totallydifferent channel conditions. When communicating on the HF, a majorityof the received noise origins from the radio channel and, thus, even twonodes quite close to one another, e.g. a distance of only a few miles orhundreds of yards, could experience completely different radio spectradue to local interference and noise sources. If the network node 10would select a frequency block on the basis of the channel measurementsmade in block 600, the network node 10 could select a frequency blockproviding poor SNR conditions at the network node 12. Therefore, it isadvantageous for the network node 10 to utilize channel informationreceived from the network node 12.

Referring to FIG. 7, the network node 12 may carry out block 600,perform the radio channel measurements and computes, on the basis of themeasurements, the lowest noise level values N_(k) ¹² for each of aplurality of frequency blocks 1 to K, thus acquiring values N₁ ¹² toN_(K) ¹². The network node 12 may then transmit the values N₁ ¹² toN_(K) ¹² to the network node 10 in step 700. Block 600 and step 700 maybe carried out regularly for every frequency block, e.g. periodically.The values N₁ ¹² to N_(K) ¹² may be transmitted in a control message ofstep S10, for example. In an embodiment, the node 12 transmits thecontrol message in every frequency block it has observed as potentialfor reception of a message. A frequency block may be determined to bepotential for reception of a message, if the lowest noise level computedfor the frequency block is below a determined threshold. In otherembodiments, the control message may be transmitted in all frequencyblocks, or another rule for selecting the frequency blocks fortransmission of the control message may be employed.

In an embodiment, each control message indicates the lowest noise levelfor the frequency block in which the control message is transmitted. Inan embodiment, each control message indicates the lowest noise level foronly the frequency block in which the control message is transmitted. Inan embodiment, the network node may indicate with the control messagegenerated and transmitted in step 700 a preferred sub-band within thefrequency block conveying the control message. As described inconnection with FIG. 6, the network node 12 may insert into the controlmessage an information element indicating preferred sub-band(s) withinthe selected frequency block. A bitmap may be used to indicate thepreferred sub-bands.

The network node 10 may receive the control message(s) indicating thelowest noise levels for the frequency blocks in 700 and, upon measuringthe signal strength of one or more radio signals received from thenetwork node in frequency blocks in step 602, the network node 10 maycompute the potential SNR for the frequency blocks and for a message tobe transmitted from the network node 10 to the network node 12 in block704 as:

${{SNR}_{1}^{10\rightarrow 12} = \frac{P_{1}^{rx}}{N_{1}^{12}}},{{SNR}_{2}^{10\rightarrow 12} = \frac{P_{2}^{rx}}{N_{2}^{12}}},\ldots\mspace{14mu},{{SNR}_{K}^{10\rightarrow 12} = \frac{P_{k}^{rx}}{N_{K}^{12}}}$

In this case, the network node 10 may assume the radio channel to bereciprocal in the sense that a radio signal transmitted from the networknode 12 to the network node 10 attenuates in a similar manner as a radiosignal transmitted from the network node 10 to the network node 12.Accordingly, the network node may employ the signal strength of a signalreceived in step 602 from the network node 12 in computation of thepotential SNR at the network node 12 for a signal transmitted by thenetwork node 10. The network node 10 may compute the potential SNR forevery frequency block for which the noise value has been received instep 700. This potential SNR may also be called reversed potential SNRin the sense that the network node 10 computes the potential SNR for theradio environment of another network node, i.e. the network node 12.

The frequency block providing the highest SNR_(k) ^(10→12) may beselected in block 704. Upon selecting the frequency block, the networknode may generate a message for transmission to the network node 12 andtransmit the message on a sub-band of the selected frequency block tothe network node 12 in step 706. The sub-band may be selected by usingthe information on the preferred sub-bands in the selected frequencyblocks, as indicated by the network node 12 in the control message(s)exchanged in step 700 or S10.

Referring to the embodiment of FIG. 4, the indication of the sub-band inblock 406 is the selection of the preferred sub-band(s) for the messagetransmitted in step 706, and block 408 may comprise the transmission ofthe message in step 706. Accordingly, block 408 comprises causing thetransmission of the message from the network node 10.

In an embodiment, the message transmitted in step 706 is the RTS messagedescribed above in connection with S12 of FIG. 2. The RTS may beaddressed to a network node with which the network node 10 wishes toinitiate data transmission.

In an embodiment, the message transmitted in step 706 is the controlmessage described above in connection with S10 of FIG. 2. The networknode 10 may keep a list of other network nodes of the wireless networkand select the frequency block and the sub-band for the control messagesuch that the probability of receiving the control messages correctly inthe network nodes of the wireless network is improved. This may beachieved by selecting the frequency block which indicates the bestpotential SNR for one or more receivers of the control message(s)transmitted by the network node 10.

FIG. 8 illustrates an embodiment that combines the embodiments of FIGS.6 and 7. FIG. 8 also illustrates system level operation in greaterdetail than FIGS. 6 and 7 that illustrate the operation mainly from theperspective of the network node 10. Referring to FIG. 8, both or allnetwork nodes of the wireless network may be configured to perform block600: measure the radio channel and determine lowest noise levels forfrequency blocks 300, 302 of the wireless networks. Block 600 may becarried out periodically.

All the network nodes may also carry out step 700 in connection with theexchange of the control messages of S10. As described above, eachcontrol message may indicate the lowest noise level for the frequencyblock in which the control message is transmitted and from theperspective of the transmitter of the control message. In someembodiments, a control message may comprise a bundle of lowest noiselevel values for a plurality of frequency blocks. Each of the controlmessages exchanged in step 700 may further carry information onpreferred sub-bands of the frequency block in which the control messagewas transmitted, e.g. a bitmap described below. The exchange of thecontrol messages further enables measuring the received signal strengthsin step 602. On the basis of the information gained in block 600 andreceived in steps 700 and 602, the network node 10 (and the othernetwork nodes) gain information that enables computation of thepotential SNR metrics described above and, furthermore, gain informationon preferred sub-bands of the other network nodes.

In block 604, the network nodes 10, 12 may compute the potential SNRmetrics for the frequency blocks and select a frequency block preferredfor reception of messages. The preferred frequency block may be storedin a memory of the respective network nodes 10, 12. Upon determining toinitiate communication with the network node 12, e.g. for the purpose oftransmitting data to the network node 12, the network node 10 computesin block 704 the reversed SNR metrics for the frequency blocks andselects a frequency block the network node 12 is deemed to provide thebest potential SNR at the network node 12, e.g. by selecting thefrequency block associated with the highest reversed SNR metric.

In step 800, the network node 10 generates a message (an RTS message)and inserts into the message an information element indicating thefrequency block selected by the network node 10 in block 604. Thenetwork node 10 may also include in the message information on thepreferred sub-band(s) or a selected sub-band within the frequency blockselected by the network node 10 in block 604. Then, the network node 10transmits the message on a sub-band of the frequency block selected inblock 704. The sub-band may be selected by using the information on thepreferred sub-bands of the selected frequency block received in step700.

Upon receiving the message in step 800, the network node 12 may extractthe contents of the message and generate a response to the message (aCTS message). The network node 12 may insert into the response messagean information element indicating the frequency block selected by thenetwork node 12 in block 604. The network node 12 may also include inthe message information on the preferred or selected sub-band(s) withinthe frequency block selected by the network node 12 in block 604. Then,the network node 12 transmits in step 802 the response message on asub-band of the frequency block indicated by the network node 10 in theinformation element contained in the message received in step 800. Thenetwork node 12 may select the sub-band by using the information on thepreferred sub-bands of the frequency block indicated in the received RTSmessage in step 800. In an alternative embodiment where the RTS messagereceived in step 800 allocates a single sub-band for the responsemessage, the network node 12 may select the sub-band allocated in theRTS message. In yet another embodiment where the RTS message specifiesno sub-band, the network node 12 may use the information on preferredsub-bands as received in step 700 select the sub-band on the basis ofthe information on the preferred sub-bands of the indicated frequencyblock.

In an embodiment, the response message indicates a plurality offrequency blocks for the data transmission. For example, the messagetransmitted in step 800 may request for a certain amount of datatransmission resources for the transmission of the data. The networknode 12 may use the request to determine the number of sub-bands and thenumber of frequency blocks needed to comply with the request, and selectthe frequency blocks in block 604. The network node 12 may then insertinto the response message of step 802 multiple information elements thatindicate the selected frequency blocks.

Upon receiving the response message in step 802, the network node 10 mayextract the contents of the response message and determine whether ornot the response message (CTS) approves the data transmission from thenetwork node 10 to the network node 12. Upon determining that the datatransmission may be commenced, the network node 10 may generate a datamessage. The network node 10 may insert into the data message payloaddata and transmit the data message in step 804. The network node 10 maytransmit the data message on one or more sub-bands of the one or morefrequency blocks indicated by the network node 12 in the informationelement contained in the response message received in step 802. In anembodiment where the network node has allocated one or more sub-band(s)for the data transmission and specified them in the CTS message, thenetwork node 10 selects the allocated sub-band(s). In anotherembodiment, the network node 10 may select the sub-band(s) by using theinformation on the preferred sub-bands of the indicated frequencyblock(s) received in step 802 or use the information on the preferredsub-bands of the indicated frequency block(s) received in step 700.

In an embodiment, the data message carries the information elementindicating the frequency block selected by the network node 10 in block604. The network node 10 may also include in the message information onthe preferred sub-band(s) within the frequency block selected by thenetwork node 12 in block 604. This frequency block and the sub-band maybe used by the network node 12 for transmitting an acknowledgementmessage for the data message. However, in another embodiment, themessage of step 800 readily indicates the preferred or selectedfrequency block of the network node 10, and the network node 12 mayselect the frequency block and the sub-band of the frequency block forthe acknowledgment message on the basis of the information received instep 800. In other words, the RTS message of step 800 may allocated thefrequency block and the sub-band for the acknowledgment message.

FIGS. 9A to 9C illustrate embodiments of the messages transferred in thewireless network. FIG. 9A illustrates an example of the control messagecommunicated in steps S10, 602, and 700. The control message maycomprise a header carrying a pilot sequence 900. The pilot sequence 900may be used to measure the signal strength in step 602, for example. Thepilot sequence may be used for other purposes as well, such as forchannel estimation used in equalization in a receiver of the controlmessage. The control message may further comprise information elements902 such as an identifier (ID) of a transmitter of the control message,the lowest noise level N_(k) for one or more frequency blocks measuredby the transmitter, as described in connection with step 700. Thecontrol message may further carry an information element indicatingexplicitly preferred sub-bands of one or more frequency blocks, e.g. thefrequency block carrying the control message. Table 1 below illustratesan example of the information element in the form of a bitmap:

TABLE 1 Sub-band index: 1 2 3 . . . K Preferred (Yes/No) 1 0 0 . . . 1The information element may consists of the K bits indicating thepreference for each sub-band in the form of binary value. The order ofthe K bits is associated with the order of the sub-bands in thefrequency block, from the lowest frequency towards the highest frequencyor vice versa. A first binary value (e.g. “1”) may indicate that asub-band is preferred and the opposite binary value (e.g. “0”) mayindicate that a sub-band is not preferred.

The bitmap or another information element indicating the preferredsub-band(s) other than the one carrying the control message providesversatility to the selection of the frequency blocks and sub-bands.Since the transmitter of the control message does not need to indicatethe preferred sub-band by transmitting the control message on thepreferred sub-band, the transmitter may select the frequency block byusing the reversed potential SNR metric and/or the sub-band by using abitmap received from another network node. In this manner, thetransmitter may improve the probability of correct reception of thecontrol message in the other network node(s). The control messages maybe considered as pilot messages that each network node transmitsperiodically or in another regular manner.

FIG. 9B illustrates an embodiment of the RTS message. The RTS messagemay also carry a pilot sequence 910 that may be used to measure thesignal strength in step 602, for example. The RTS message may furthercomprise information elements 912 such as an identifier of thetransmitter of the RTS message, an indicator for the frequency blockselected in block 604 and, optionally, a bitmap of preferred sub-bandsfor the frequency block selected in block 604. In another embodiment,the RTS message may comprise a bitmap of preferred sub-bands for thefrequency block carrying the RTS message. The RTS message may furtherinclude an information element explicitly specifying a sub-band of theindicated frequency block that shall be used for transmitting theresponse to the RTS message (the CTS message). This information elementmay serve also as an indicator for the sub-band of the acknowledgmentmessage related to a subsequent data transmission. The bitmap containedin the RTS message is not necessarily used for selecting a sub-band forthe response to the RTS message. Any node may be capable of receivingthe RTS message so the bitmap may serve the same purpose as in thecontrol message in step 700. The RTS message may further carry aninformation element that indicates an amount of data transmissionresources requested by the network node transmitting the RTS message. Arecipient of the RTS message may use this information in the selectionand allocation of a number of sub-bands for the data transmission.

FIG. 9C illustrates an embodiment of the CTS message. The CTS messagemay also carry a pilot sequence 920. The CTS message may furthercomprise information elements 922 such as an indicator for the frequencyblock selected in block 604 for the data transmission and an indicatorfor one or more sub-bands of the selected frequency block allocated fordata transmission. The bitmap may be omitted from the CTS message.

As indicated above in connection with FIG. 5, the lowest noise levelestimated for a frequency block may be associated with a sub-banddifferent from a sub-band on which the radio signal used for measuringthe P_(rx) is received. The sub-bands may naturally be the same but thesub-bands need not to be the same for the estimation of the potentialSNR. In an embodiment, a criterion may be defined that the sub-bandsshall belong to the same frequency block. Compared with someconventional methods, a purpose of the calculation of the potential SNRis not to estimate a SNR for a frequency band where a pilot/referencesignal is received but to find a sub-band where the SNR is the highestwithout requiring transmission of the pilot/reference signal on allcandidate sub-bands.

FIG. 10 illustrates a process for estimating the potential SNR as anembodiment of block 604 or 704. Referring to FIG. 10, an apparatussuitable for a network node 10 to 12 receives a signal on a sub-band ofa frequency block k and measures the received signal strength P_(k)^(rx) from the received signal. As described above, the signal may bereceived in S10, 602, 606 (by the network node 10), 700, or 800 (by thenetwork node 12). In block 1002, the apparatus computes the potentialSNR value SNR_(k) for the frequency block k on the basis of the receivedsignal strength value P_(k) ^(rx) computed in block 1000 and the lowestnoise level value available for the frequency block k. The lowest noiselevel value is in this embodiment associated with a different sub-bandthan the sub-band where the signal was received and for which thereceived signal strength P_(k) ^(rx) is measured. The lowest noise levelmay, however, be present on the sub-band where the received signalstrength was measured. A similar procedure may be carried out for everyfrequency block where a node 10/12 has received a radio signal from theother node 12/10 of the wireless network.

FIG. 11 illustrates an embodiment of an apparatus comprising means forcarrying out the functionalities of the network node according to anyone of the above-described embodiments. The apparatus may be a radiocommunication apparatus implemented as a portable device, e.g. acomputer (PC), a laptop, a tabloid computer, a portable radio phone, amobile radio platform (installed to a vehicle such as a truck or aship), or any other apparatus provided with radio communicationcapability. In some embodiments, the apparatus is the vehicle equippedwith the radio communication capability. In other embodiments, theapparatus is a fixed station, e.g. a base station. In furtherembodiments, the apparatus is comprised in any one of theabove-mentioned apparatuses, e.g. the apparatus may comprise acircuitry, e.g. a chipset, a processor, a micro controller, or acombination of such circuitries in the apparatus.

The apparatus may comprise a communication controller circuitry 10configured to control the communications in the apparatus. Thecommunication controller circuitry 10 may comprise a control messageprocessor 12 handling control signalling communication with respect toestablishment, operation, and termination of the radio connections. Thecontrol message processor 12 may also carry out any other controlfunctionalities related to the operation of the radio links, e.g.transmission, reception, and extraction of the control messages in stepsS10, 602, 700, and the RTS/CTS messages in steps S12, S14, 606, 608,706, 800, 802. The communication controller circuitry 10 may furthercomprise a data message processor 14 that handles transmission andreception of payload data over the radio links, e.g. in steps S15, 804.The communication controller circuitry 10 may further comprise a channelselection circuitry 16 configured to select a preferable frequency blockand/or a preferable sub-band for various messages transmitted and/orreceived by the apparatus. The channel selection circuitry 16 mayexecute the process of FIG. 4 or any one of its embodiments, forexample.

The channel selection circuitry 16 may comprise a frequency blockselection circuitry 17 configured to select a frequency block for amessage transmitted/received by the apparatus. For a message to betransmitted from the apparatus, the frequency block selection circuitry17 may first determine whether or not there has already been allocated afrequency block to the message. The determination may comprise checkingwhether or not the apparatus has received a message through the controlmessage processor 12 that indicates the frequency block for the message.An example of such a situation is that the apparatus has received an RTSmessage that allocates the frequency block and a sub-band for aresponding CTS message the apparatus is currently preparing for thetransmission. In such a case, the frequency block selection circuitry 17may select the frequency block indicated in the received RTS message. Ifno allocation of the frequency block has been made, the frequency blockselection circuitry may compute or determine the reversed potential SNRvalues according to block 704 for an intended recipient of the messageand select a frequency block, as described above in connection withblock 704. Upon selecting the frequency block, the frequency blockselection circuitry 17 may output information on the selected frequencyblock to a sub-band selection circuitry 18 configured to select asub-band of the frequency block for the message. The sub-band selectioncircuitry 18 may employ information on the preferred sub-bands receivedfrom the intended recipient of the message, e.g. the bitmap of Table 1,or received allocation information allocating a sub-band fortransmission. Upon selecting the frequency block and the sub-band, thechannel selection circuitry 16 may output information on the selectionto either control message processor 12 or the data message processor 14,depending on whether the message is a control message or a data message,and the respective processor 12, 14 may carry out transmission of themessage.

The circuitries 12 to 18 of the communication controller circuitry 10may be carried out by the one or more physical circuitries orprocessors. In practice, the different circuitries may be realized bydifferent computer program modules. Depending on the specifications andthe design of the apparatus, the apparatus may comprise some of thecircuitries 12 to 18 or all of them.

The apparatus may further comprise a memory 20 that stores computerprograms (software) 22 configuring the apparatus to perform theabove-described functionalities of the network node. The memory 20 mayalso store communication parameters and other information needed for theradio communications. For example, the memory may store a list ofpreferred sub-bands, received signal strength values, and lowest noiselevel values of frequency blocks for each network node the apparatus hasdetected or has capability of communicating with. The memory 20 mayserve as a buffer for data packets to be transmitted. The apparatus mayfurther comprise radio interface components 26 providing the apparatuswith radio communication capabilities with other network nodes. Theradio interface components 26 may comprise standard well-knowncomponents such as amplifier, filter, frequency-converter,analog-to-digital (A/D) and digital-to-analog (D/A) converters,(de)modulator, and encoder/decoder circuitries and one or more antennas.The apparatus may further comprise a user interface enabling interactionwith the user. The user interface may comprise a display, a keypad or akeyboard, a loudspeaker, a smartcard and/or fingerprint reader, etc.

As used in this application, the term ‘circuitry’ refers to all of thefollowing: (a) hardware-only circuit implementations, such asimplementations in only analog and/or digital circuitry, and (b) tocombinations of circuits and software (and/or firmware), such as (asapplicable): (i) a combination of processor(s) or (ii) portions ofprocessor(s)/software including digital signal processor(s), software,and memory(ies) that work together to cause an apparatus to performvarious functions, and (c) to circuits, such as a microprocessor(s) or aportion of a microprocessor(s), that require software or firmware foroperation, even if the software or firmware is not physically present.

This definition of ‘circuitry’ applies to all uses of this term in thisapplication. As a further example, as used in this application, the term“circuitry” would also cover an implementation of merely a processor (ormultiple processors) or portion of a processor and its (or their)accompanying software and/or firmware. The term “circuitry” would alsocover, for example and if applicable to the particular element, abaseband integrated circuit or applications processor integrated circuitfor a mobile phone or a similar integrated circuit in server, a cellularnetwork device, or other network device.

In an embodiment, the apparatus carrying out the embodiments of theinvention in the communication apparatus comprises at least oneprocessor and at least one memory including a computer program code,wherein the at least one memory and the computer program code areconfigured, with the at least one processor, to cause the apparatus tocarry out the steps of any one of the processes of FIGS. 2 to 10.Accordingly, the at least one processor, the memory, and the computerprogram code form processing means for carrying out embodiments of thepresent invention in the communication apparatus.

The processes or methods described in FIGS. 2 to 10 may also be carriedout in the form of a computer process defined by a computer program. Thecomputer program may be in source code form, object code form, or insome intermediate form, and it may be stored in some sort of carrier,which may be any entity or device capable of carrying the program. Suchcarriers may include transitory or non-transitory media such as a recordmedium, computer memory, read-only memory, electrical carrier signal,telecommunications signal, and software distribution package, forexample. Depending on the processing power needed, the computer programmay be executed in a single electronic digital processing unit or it maybe distributed amongst a number of processing units.

The present invention is applicable to radio communication systemsdefined above but also to other suitable telecommunication systems. Theprotocols used, the specifications of mobile telecommunication systems,their network elements and subscriber terminals, develop rapidly. Suchdevelopment may require extra changes to the described embodiments.Therefore, all words and expressions should be interpreted broadly andthey are intended to illustrate, not to restrict, the embodiment. Itwill be obvious to a person skilled in the art that, as technologyadvances, the inventive concept can be implemented in various ways. Theinvention and its embodiments are not limited to the examples describedabove but may vary within the scope of the claims.

The invention claimed is:
 1. A method for selecting a frequency blockfor radio communications, the method comprising in a first node of awireless network: determining a first value representing a lowest noiselevel of a radio channel at a second node in a first frequency block;calculating a first metric representing a ratio between a first desiredsignal strength and the first value, wherein the first metric representsthe highest potential signal-to-noise ratio in the first frequency blockand the first desired signal is measured on a frequency different fromthe frequency of the lowest noise level; determining a second valuerepresenting a lowest noise level of a radio channel at the second nodein a second frequency block; calculating a second metric representing aratio between a second desired signal strength and the second value,wherein the second metric represents the highest potentialsignal-to-noise ratio in the second frequency block; selecting, on thebasis of at least the first and second metric and amongst a set offrequency blocks comprising at least the first and second frequencyblock, a frequency block for transmission of a message between the firstnode and the second node; indicating a sub-band of the selectedfrequency block for the message; and causing transmission of the messageon the indicated sub-band of the selected frequency block, wherein thefirst value and the second value are received at the first node from thesecond node and represent the lowest noise level of the radio channel atthe second node.
 2. The method of claim 1, wherein the first metric iscalculated on the basis of the first value and a reception signalstrength of a first radio signal received at the first node from thesecond node, and the second metric is calculated on the basis of thesecond value and the reception signal strength of a second radio signalreceived at the first node from the second node.
 3. The method of claim1, wherein said selecting the frequency block is performed for thetransmission of said message that is transmitted from the first node tothe second node.
 4. The method of claim 3, further comprising:determining a third value representing a lowest noise level of a radiochannel at the first node in a third frequency block; calculating athird metric representing a ratio between the third value and areception signal strength of a third radio signal received at the firstnode from the second node, wherein the third metric represents thehighest potential signal-to-noise ratio in the third frequency block;determining a fourth value representing a lowest noise level of a radiochannel at the first node in a fourth frequency block; calculating afourth metric representing a ratio between the fourth value and thereception signal strength of a fourth radio signal received at the firstnode from the second node, wherein the fourth metric represents thehighest potential signal-to-noise ratio in the fourth frequency block;and selecting, on the basis of at least the third and fourth metric andamongst a set of frequency blocks comprising at least the third andfourth frequency block, a frequency block for transmission of a thirdmessage from the second node to the first node.
 5. The method of claim4, further comprising: transmitting to the second node a fourth messagecomprising an information element indicating the selected frequencyblock for said third message; and receiving the third message from thesecond node in the frequency block indicated in the fourth message. 6.The method of claim 1, wherein said indicating the sub-band of theselected frequency block for the message comprises selecting thesub-band, wherein the selection is based on information on sub-bandsthat are deemed to be free of colliding transmissions at a receiver ofthe message.
 7. The method of claim 6, wherein the information isreceived in the first node from the second node as a bitmap, wherein thebitmap indicates for each sub-band of at least said first and secondfrequency block, whether or not said each sub-band is preferred by thesecond node for transmission of messages from the first node to thesecond node.
 8. An apparatus comprising: at least one processor and atleast one memory including a computer program code, wherein the at leastone memory and the computer program code are configured, with the atleast one processor, to cause the apparatus to perform operationscomprising: determining a first value representing a lowest noise levelof a radio channel in a first frequency block at a second node of awireless network; calculating a first metric representing a ratiobetween a first desired signal strength and the first value, wherein thefirst metric represents the highest potential signal-to-noise ratio inthe first frequency block and the first desired signal is measured on afrequency different from the frequency of the lowest noise level;determining a second value representing a lowest noise level of a radiochannel at the second node in a second frequency block; calculating asecond metric representing a ratio between a second desired signalstrength and the second value, wherein the second metric represents thehighest potential signal-to-noise ratio in the second frequency block;selecting, on the basis of at least the first and second metric andamongst a set of frequency blocks comprising at least the first andsecond frequency block, a frequency block for transmission of a messagebetween a first node and the second node; indicating a sub-band of theselected frequency block for the message; and causing transmission ofthe message on the indicated sub-band of the selected frequency block,wherein the first value and the second value are received by theapparatus from the second node and represent the lowest noise level ofthe radio channel at the second node.
 9. The apparatus of claim 8,wherein the at least one memory and the computer program code areconfigured, with the at least one processor, to cause the apparatus toperform operations comprising: calculating the first metric on the basisof the first value and a reception signal strength of a first radiosignal received at the first node from the second node; and calculatingthe second metric on the basis of the second value and the receptionsignal strength of a second radio signal received at the first node fromthe second node.
 10. The apparatus of claim 8, wherein the at least onememory and the computer program code are configured, with the at leastone processor, to cause the apparatus to perform said selecting thefrequency block for the transmission of said message that is transmittedfrom the first node to the second node.
 11. The apparatus of claim 10,wherein the at least one memory and the computer program code areconfigured, with the at least one processor, to cause the apparatus toperform operations comprising: determining a third value representing alowest noise level of a radio channel at the first node in a thirdfrequency block; calculating a third metric representing a ratio betweenthe third value and a reception signal strength of a third radio signalreceived at the first node from the second node, wherein the thirdmetric represents the highest potential signal-to-noise ratio in thethird frequency block; determining a fourth value representing a lowestnoise level of a radio channel at the first node in a fourth frequencyblock; calculating a fourth metric representing a ratio between thefourth value and the reception signal strength of a fourth radio signalreceived at the first node from the second node, wherein the fourthmetric represents the highest potential signal-to-noise ratio in thefourth frequency block; and selecting, on the basis of at least thethird and fourth metric and amongst a set of frequency blocks comprisingat least the third and fourth frequency block, a frequency block fortransmission of a third message from the second node to the first node.12. The apparatus of claim 11, wherein the at least one memory and thecomputer program code are configured, with the at least one processor,to cause the apparatus to perform operations comprising: transmitting tothe second node a fourth message comprising an information elementindicating the selected frequency block for said third message; andreceiving the third message from the second node in the frequency blockindicated in the fourth message.
 13. The apparatus of claim 8, whereinsaid indicating the sub-band of the selected frequency block for themessage comprises selecting the sub-band, wherein the selection is basedon information on sub-bands that are deemed to be free of collidingtransmissions at a receiver of the message.
 14. The apparatus of claim13, wherein the at least one memory and the computer program code areconfigured, with the at least one processor, to cause the apparatus toperform operations comprising receiving the information in the firstnode from the second node as a bitmap, wherein the bitmap indicates foreach sub-band of at least said first and second frequency block, whetheror not said each sub-band is preferred by the second node fortransmission of messages from the first node to the second node.
 15. Acomputer program product embodied on a non-transitory distributionmedium readable by a computer and comprising program instructions which,when loaded into the computer, execute a computer process in a firstnode of a wireless network, the computer process comprising: determininga first value representing a lowest noise level of a radio channel at asecond node in a first frequency block; calculating a first metricrepresenting a ratio between a first desired signal strength and thefirst value, wherein the first metric represents the highest potentialsignal-to-noise ratio in the first frequency block and the first desiredsignal is measured on a frequency different from the frequency of thelowest noise level; determining a second value representing a lowestnoise level of a radio channel at the second node in a second frequencyblock; calculating a second metric representing a ratio between a seconddesired signal strength and the second value, wherein the second metricrepresents the highest potential signal-to-noise ratio in the secondfrequency block; selecting, on the basis of at least the first andsecond metric and amongst a set of frequency blocks comprising at leastthe first and second frequency block, a frequency block for transmissionof a message between the first node and the second node; indicating asub-band of the selected frequency block for the message; and causingtransmission of the message on the indicated sub-band of the selectedfrequency block, wherein the first value and the second value arereceived at the first node from the second node and represent the lowestnoise level of the radio channel at the second node.